GaAsSb is attracting attention as a compound semiconductor used as a base layer material of an InP-based heterostructure bipolar transistor (HBT). GaAsSb lattice-matches an InP substrate by a composition GaAs(0.51)Sb(0.49). An InP/GaAs(0.51)Sb(0.49)/InP-based HBT using GaAsSb as a base can realize good high-frequency characteristics and high-breakdown-voltage characteristics at the same time (reference 1: “300 GHz InP/GaAsSb/InP Double HBTs with High Current Capability and BVceo≧6 V”, M. W. Dvorak, Student Member, IEEE, C. R. Bolognesi, Member, IEEE, O. J. Pitts, and S. P. Watkins, Member, IEEE,: IEEE ELECTRON DEVICE LETTERS, VOL. 22, NO. 8, AUGUST 2001 p. 361). Heterostructures include a type-I heterostructure in which a conduction band ECA and valence band EVA of a semiconductor A and a conduction band ECB and valence band EVB of a semiconductor B have potentials (energy potentials) represented by “ECA>ECB” and “EVA>EVB” as shown in FIG. 14, and a type-II heterostructure in which “ECA<ECB” and “EVA<EVB” as shown in FIG. 15. The heterostructure bipolar transistor described above is made of the type-II heterostructure.
When GaAs(0.51)Sb(0.49) is used as a base, the potential of this conduction band edge becomes higher than that of the conduction band edge of an InP collector layer, and this eliminates the current blocking effect in the collector which is a problem in an InGaAs/InP-based HBT. As described above, therefore, good high-frequency characteristics and high-breakdown-voltage characteristics can be obtained at the same time. In addition, when GaAsSb is applied to a base layer, carbon atoms having a small diffusion coefficient can be doped at a high concentration, and this is advantageous in reducing the base parasitic resistance.
Even when InGaAs is applied to a base layer, carbon atoms can be used as a p-type dopant, but ultrahigh-concentration doping at 5×1019 cm−3 or more is difficult. Also, it is pointed out that when an InGaAs layer is to be formed by metal organic chemical vapor deposition (MOCVD) as a general growth method, carbon acceptors are passivated by hydrogen atoms. This passivation of the carbon acceptors by hydrogen not only deteriorates the characteristics of a device by increasing the resistance of the base layer, but also causes a burn-in effect by which the resistance value fluctuates because the ratio of the carbon acceptors passivated by hydrogen changes while an electric current is applied, thereby deteriorating the reliability of the device as well. GaAsSb has a very high resistance against this hydrogen passivation.